CMake packaging doesn't support example usage

[HunterZ]

The project readme says to use #include <ixwebsocket/xyz.h> but the CMake install logic uses .../include/ixwebsocket in the exported target properties: https://github.com/machinezone/IXWebSocket/blob/c5a02f1066fb0fde48f80f51178429a27f689a39/CMakeLists.txt#L275

This means that unless you get the parent include directory via some other means, the #include directive breaks, which is exactly what just happened to me when I tried to reuse some of my ixwebsocket-dependent code in another CMake project.

Note that ixwebsocket's pkgconfig setup exports the parent include directory, so fixing this would bring those into alignment: https://github.com/machinezone/IXWebSocket/blob/c5a02f1066fb0fde48f80f51178429a27f689a39/ixwebsocket.pc.in#L4